This invention relates to high temperature stabilized mullite aluminum titanate compositions formed by doping and pre-calcining the mullite with titania and hematite, and mixing the doped, pre-calcined mullite with pre-calcined aluminum titanate. Aluminum titanate is used in applications where thermal shock and ultimate use temperatures are high, as well as for diesel particulate filters and catalytic converters. Typically, aluminum titanate is desirable in harsh thermal environments requiring high thermal shock resistance, low thermal expansion, and high mechanical strength. Like most ceramic compositions, it has long been recognized that when aluminum titanate is subjected to such thermally harsh environments, the crystalline phases of the ceramic matrix are reordered, thus impairing the desirable thermal and mechanical properties of the ceramic article. In particular, at temperatures of 800.degree.-1300.degree. C., aluminum titanate tends to decompose to its titania and alumina precursors.
Many approaches have been suggested for stabilizing aluminum titanate against decomposition at high temperatures. For example, it has been suggested that the addition of rare earth oxides and iron oxides to aluminum titanate and aluminum titanate mullite compositions will stabilize the aluminum titanate against decomposition. A major disadvantage of the above approach is that such additives tend to adversely affect the thermal and mechanical properties of the ceramic composition.
More recently, it has been suggested that aluminum titanate can be stabilized at high temperatures by partial substitution of iron for aluminum. For example, it has been suggested in U.S. Pat. No. 4,855,265 that the addition of rare earth oxides and mullite to aluminum titanate will improve the mechanical strength of the ceramic without interfering with the thermal properties of mullite aluminum titanate compositions. This reference discloses that the addition of mullite formers to pre-calcined aluminum titanate grains produces a composite material consisting essentially of a mixture of aluminum titanate and mullite grains which exhibits good mechanical strength, thermal shock resistance and durability. A major problem with the high temperature stabilized aluminum titanate compositions of the prior art is that structures formed with these compositions tend to be especially prone to cracking during the firing process.
It is therefore, the primary objective of the present invention to develop a sintered aluminum titanate article which exhibits good mechanical strength, durability and thermal shock resistance at high temperatures, and which will not crack during firing.
It is also an object of the present invention to develop a method by which large samples of mullite-aluminum titanate can be made to withstand faster firing rates than had previously been possible.